A vertical-type heat treatment furnace has come to be used as the heat treatment apparatus of silicon wafers in view of the recent trend toward larger diameter silicon wafers. A vertical wafer boat having a plurality of vertical support columns is installed in the vertical type heat treatment furnace, and the wafers are heat-treated after being inserted in a plurality of support grooves cut in the inside walls of the support columns. By this supporting method, however, each wafer is supported by wafer support parts disposed along the outermost periphery of the wafer, and the weight of the wafer is imposed on the wafer only at the vicinity of the support parts in a concentrated manner. As a consequence, a large bending stress is imposed on the wafer at the vicinity of the support parts during the high temperature heat treatment employed in the production of the SIMOX wafers, annealed wafers and the like, and, if the stress exceeds the yield stress of the silicon wafer at the temperature of the heat treatment, there is a problem that defects called slip dislocations occur inside the wafer during the heat treatment.
For solving that problem, conventionally, a method of increasing the area for supporting a wafer and thus suppressing the occurrence of the slip dislocations by inserting a wafer support plate 6 in the support grooves 3 of a plurality of support columns 2 of a wafer boat 1 and placing a wafer thereon, as seen in FIG. 21, is used. Heat treatment is then applied. Further, by forming a notch 7 concavely in the wafer support plate, it is possible to apply a wafer conveying apparatus of a system of holding a wafer on its bottom surface by a vacuum chuck and conveying it, and thus high productivity through the high-speed loading and unloading of the wafer is realized.
Even by this technology, however, the occurrence of slip dislocations is not suppressed sufficiently. In the technology, the wafer support plate deforms during the heat treatment because the wafer support plate is not symmetrical with respect to its center and is supported, by the support columns 2, asymmetrically. What is more, the frictional force between the silicon wafer and the wafer support plate increases due to an increase in the contact area between the silicon wafer and the wafer support plate. For these reasons, the stress caused by the thermal deformation of the wafer support plate is transferred to the silicon wafer more easily and thus the slip dislocations occur more easily.
To solve these problems, Japanese Unexamined Patent Publication No. 2000-91406 proposes a wafer holder wherein three silicon balls 12 acting as wafer support members are placed on a wafer support plate 11 without a notch and a silicon wafer 10 is supported at the three points by the silicon balls, as shown in FIG. 22. By this method, the thermal deformation of the wafer support plate is comparatively small because there is no notch in the wafer support plate. Further, the supporting of the wafer at the points alleviates the frictional force between the wafer and the wafer support plate when the heat treatment is done in a non-oxidizing atmosphere in which the silicon balls and the silicon wafer do not weld together. Therefore, the slip dislocations of the wafer caused by the thermal deformation of the wafer support plate can be suppressed.
Even by this technology, however, the silicon balls still weld with the silicon wafer during the heat treatment in an oxidizing atmosphere employed for the production of SIMOX annealed wafers, and the slip dislocations of the wafer caused by the thermal deformation of the wafer support plate cannot be suppressed sufficiently. Besides, as the wafer support plate does not have a notch, the wafer conveying apparatus tends to be large, and the wafer charging and discharging movements of the wafer conveying apparatus requires large spaces above and beneath the wafer support plate. As a result, the wafer conveying movements of the wafer conveying apparatus become complicated and the wafer conveying time becomes longer and, therefore, the productivity of the heat treatment furnace per batch is lowered. A yet more serious problem is that, using this technology, one third of the weight of the wafer is concentrated at a support part, and the stress resulting from the wafer weight causes the slip dislocations of the wafer at the supported portions of the wafer. In view of the recent trend of increasing the diameter of a silicon wafer and the consequent heavier wafer weight, it is clear that the problem of the slip dislocations will become more and more serious, as long as the above method of distributing the wafer weight to only three points of the wafer support parts is employed.
Methods for preventing the occurrence of the slip dislocations without using a wafer support plate have been proposed. An example is the method disclosed in Japanese Unexamined Patent Publication No. H11-40659, wherein four wafer support parts per wafer are provided in a wafer boat. In this case, since there are four support parts to bear the wafer weight, the stress caused by the weight of the silicon wafer at each of the supported portions is smaller than in the case that the wafer weight is borne by three wafer support parts of a similar shape.
By this method, however, it is necessary that the heights of all the four support parts have to be within a tolerance of 30 μm, and there are problems that it is extremely difficult to fabricate a wafer boat having such a dimensional accuracy and that it is very costly. The fabrication of a wafer boat having five or more support parts per wafer is more difficult if the support parts have to have a height accuracy equal to or better than the above. Thus, there is a problem that the number of the wafer support parts cannot be increased easily. Needless to say, the same problem still persists when the number of the silicon balls placed on the wafer support plate used in the method disclosed in the Japanese Unexamined Patent Publication No. 2000-91406 is increased to four or more.
In the heat treatment of silicon wafers, especially the high temperature heat treatment employed in the production of SIMOX wafers, annealed wafers and the like, suppressing the slip dislocations satisfactorily using an economical wafer holder without deteriorating productivity is, therefore, an important challenge to be solved for improving the production yield of the silicon wafers. What is necessary in order to solve the challenge is a wafer holder satisfying either of the following two requirements:
(1) a wafer holder is structured so as to support a wafer at points in order to alleviate the stress caused by the thermal deformation of a wafer support plate, and so as to support the wafer with four or more wafer support parts in order to suppress the stress caused by the wafer weight; and
(2) if a wafer is supported by a face or faces, the thermal deformation of a wafer support plate is small enough not to affect the wafer.
It is necessary for the wafer holder to further satisfy the following requirements:
(3) a wafer holder has a notch so as to allow the use of a wafer conveying apparatus of a system to hold a wafer on its bottom surface by a vacuum chuck and convey it in order to maintain high productivity;
(4) a wafer holder is made of a material which does not weld with a silicon wafer during heat treatment in order to prevent the occurrence of slip dislocations caused by the welding; and so forth.
In addition, a wafer boat and a heat treatment furnace incorporating the wafer holder satisfying the above requirements are necessary. However, as described above, none of conventional technologies is capable of solving the above problems.
The object of the present invention, therefore, is to solve the above problems and provide a wafer holder, a wafer support member, a wafer boat and a heat treatment furnace suitable for the high temperature heat treatment of a silicon wafer.